1. Field of the Invention
This invention relates to acoustic transducers for use in pulse-echo ranging applications.
2. Review of the Art
U.S. Pat. No. 4,333,028 (Panton) issued June 1, 1982 discloses a transducer for use in such applications which provides good performance and has received widespread commercial acceptance. The Panton invention, as set forth in claim 25 of U.S. Pat. No. 4,333,028, provides a broadly tuned directional transducer system comprising a radiating plate having a higher flexural mode resonance at substantially the operating frequency of the system, a transducer element of much smaller effective area than the plate and coupled thereto, and coupling means formed of low-loss acoustic propagation material of much lower acoustic impedance than the plate and applied to alternate antinodal zones of the radiating surface thereof such as to avoid substantial cancellation in the far field of sound radiated from said alternate antinodal zones of the plate by sound radiated from the remaining antinodal zones of the plate. Various different ways in which this invention can be implemented are described, depending variously on enhancing or reducing radiation from alternate antinodal zones as compared to adjacent antinodal zones, and/or adjusting the phase of radiation from adjacent antinodal zones to reduce or eliminate far-field cancellation. A particular advantage of the Panton transducer is that, as compared to transducers of previous designs, for example those disclosed in U.S. Pat. No. 3,674,945, (Hands), issued July 4, 1972, it utilizes very much smaller quantities of piezoelectric material, particularly in transducers operating at low frequency. This in turn permits the cost and weight of the transducer to be greatly reduced without any performance penalty. It has however been found that, in certain industrial environments involving high temperatures and/or chemically aggressive atmospheres, the low loss acoustic coupling materials utilized to couple the transducer to the atmosphere, which are usually fabricated from foamed synthetic plastics or rubbers, can be subject to unacceptably rapid deterioration in service. In some applications, this latter problem has required the use of transducers of the older design, despite the substantial cost and weight penalty.
In an endeavour to overcome the problems involved in forming the coupling means of the Panton transducer from foam materials, United Kingdom Patent Application No. 2186465A (Endress & Hauser), published Aug. 12, 1987, discloses a version of the Panton transducer as set forth above in which a grid is applied to the front of the radiating plate so as to define concentric rings and channels, the rings and channels being in front of alternate antinodal zones of the plate. The channels contain shaped bodies of air applied to the plate, which bodies provide the coupling means formed from low loss acoustic propagation material having a much lower acoustic impedance than the plate. The rings, which are not mechanically coupled to the plate, block radiation from the remaining alternate antinodal zones. Since the channels in the grid configure the air which they contain so that the latter provides the required coupling means. There is no necessity for using vulnerable foamed materials: the grid itself, which acts largely as a mask, may be made from heat and corrosion resistant material. On the other hand, the confinement of a portion of the ambient atmosphere to form the coupling means provides less than ideal coupling between the plate and the far field, making it more difficult to control ringing of the transducer. It is also difficult to ensure that material does not become lodged between the grid and the radiating plate, with severe effects upon the performance of the transducer, whilst multiple reflections between the radiating plate and the grid may also degrade transducer performance.
It is known to increase the effective area of an axial mode transducer by applying to its front surface a frustoconical radiating plate and a loading plate: U.S. Pat. No. 4,183,007 (Baird) issued June 8, 1980 is exemplary of such transducers. There are however fairly severe limits upon the extent to which the size of the radiating surface can be extended in this manner, since the periphery of the radiating plate will commence to produce flexural mode responses with deleterious effects on transducer performance and polar response.
Besides the Hands patent mentioned above, various proposals have been made for transducer assemblies comprising multiple transducer arrays in which the transducers are operated in unison or near unison in order to provide the effect of a single much larger transducer, and/or to enable manipulation of the polar radiation pattern of the transducer. Examples of such transducers are disclosed in U.S. Pat. Nos. 2,567,407 (Slaymaker), 4,122,725 (Thompson), and 4,211,948 (Smith et al). Although such array may be provided with common matching layers, the transducers operate essentially independently, and a large quantity of piezoelectric material is required. U.S. Pat. No. 2,406,767 shows, in FIG. 10, an array of closely adjacent piezoelectric transducers submerged in liquid between front and rear plates. Shear effects in the liquid together with the closeness of the transducers are relied upon to maintain phase coherence and piston like operation of the plates. Again, a large quantity of piezoelectric material is required, the elements having together substantially the same area as that of the radiating plate of the transducer.
It is also known to provide the matching layer of a transducer with a protective membrane: in addition to the Hands patent already mentioned, exemplary arrangements are shown in U.S. Pat. No. 4,297,607 (Lynnworth et al), who also show in FIG. 5 a multitransducer array of the type already discussed, and U.S. Pat. No. 4,523,122 (Tone) (see FIG. 2).